This invention relates to an actuating apparatus and an actuator.
There is a demand for actuators for electric driving control, having a high torque at a low speed and readily controlled in speed and phase. Conventional actuators were combinations of a d.c. motor, or an a.c. motor, and a reduction gear for stepping down the motor output speed.
In such arrangements using a reduction gear system, the d.c. motor, or a.c. motor, has a speed of rotation as high as several thousands r.p.m. which causes the reduction gear to be worn out in a relatively short time of use, resulting in a short life of about 3,000 hours. Also, d.c. motors including a brush typically have short lives because the brush is worn out.
Brushless motors using a permanent magnet as a rotor and movable-magnet motors are long-life actuators that can rotate at relatively low speeds. In a brushless motor, the rotor can be in a desired direction at a desired speed by applying to a field coil of a stator a pulsating current inverted in polarity in synchronism with the rotation phase of the rotor. A magnetic sensor such as a Hall element is used as rotation detecting means to detect the rotating phase of the rotor such that a resultant detection output is used to switch the current to be applied to the field coil.
FIG. 2 shows a cross-sectional view of one of such prior typical brushless motors. A cylindrical yoke 10 accommodates a bobbin 14 on which a field coil 12 is wound. A rotor 16 in the form of a cylindrical (or disk-like) permanent magnet is held in the cylindrical interior space of the bobbin 14. The rotor 16 is spaced apart from the bobbin 14 by a constant distance. A Hall element 18 is located at an appropriate circumferential position of the bobbin 14, which may be a position where it intersects with the coil plane of the field coil 12, and it is oriented such that the sensitive direction coincides with the exciting direction of the field coil 12.
FIG. 3 is a diagram showing torque output characteristics and Hall element output characteristics with a constant voltage being applied to the field coil 12. The torque output characteristics and the Hall element output characteristics exhibit clean sinusoidal waveforms. As shown in FIG. 3, the output characteristics of the Hall element 18 and the torque characteristics are different in phase by 90 degrees. Since the torque becomes negative at the point just beyond 180 degrees, the rotor 16 can be continuously rotated in the same direction by applying the opposite-polarity voltage to the field coil 12. FIG. 4 shows a waveform diagram of the output torque characteristics and the voltage applied to the field coil 12 during rotation in one direction. In FIG. 4, (1) is the output torque characteristics, and (2) is the voltage to the field coil 12. The output torque characteristics represent the absolute value of a sinusoidal wave.
It is desired that an actuator has a long life not less than 10,000 hours at a low speed of rotation and can move quietly and smoothly. It is also desired for an actuator to be precisely controlled in speed and position. In this regard, it is desired that a wide speed range can be realized without using a reduction gear and that the output torque includes no or quite a few ripple components. However, no existing apparatus satisfies the requirements.
It is therefore an object of the invention to provide an actuating apparatus and an actuator which satisfy the requirements referred to above.